les: extend error message for coinbase API calls (#19380)

les/backend.go

@@ -193,12 +193,12 @@ type LightDummyAPI struct{}
 
 // Etherbase is the address that mining rewards will be send to
 func (s *LightDummyAPI) Etherbase() (common.Address, error) {
-	return common.Address{}, fmt.Errorf("not supported")
+	return common.Address{}, fmt.Errorf("mining is not supported in light mode")
 }
 
 // Coinbase is the address that mining rewards will be send to (alias for Etherbase)
 func (s *LightDummyAPI) Coinbase() (common.Address, error) {
-	return common.Address{}, fmt.Errorf("not supported")
+	return common.Address{}, fmt.Errorf("mining is not supported in light mode")
 }
 
 // Hashrate returns the POW hashrate